It seems logical that a car with a full battery is no heavier than when it is empty. Mobile phones also seem to always weigh exactly the same. And yet fully charged batteries may increase the weight of electric cars.
Conventional cars with a full tank are heavier than empty ones. Since a liter of petrol weighs about a kilo, with a full tank you simply have the weight of an extra passenger on board, which in turn has an influence on consumption. But what about electric cars? After all, charging and discharging only initiates chemical processes, but no material is transported in or out of the batteries.
However, since Albert Einstein, we have known that energy, like light, has mass. That is, it has a weight. Decisive here is the best known and at the same time probably the most infamous formula in the world E=mc². It determines that energy has weight, and that also applies to the electricity stored in the battery of an electric car.
333 million Tesla Model S
But how much? The battery of a Tesla Model S weighs about 750 kilograms empty. According to website 24Auto.de it should be about three nanograms heavier when fully charged. A nanogram is one billionth of a gram. This makes 333 million fully charged Tesla Model S about a gram heavier than the same number of empty Teslas.
However, Auke Hoekstra, expert in the field of electric cars and affiliated with Eindhoven University of Technology, wonders whether Einstein’s formula can be used in this. “And even then it’s not right. One gram is about 90 million megajoules, which is 25 million kWh and that is 25,000 fully charged Tesla Model S with 100 kWh.” Hoekstra thinks that ions are mainly shifted when charging and discharging a battery and wonders whether this causes a change in weight.
Theory and practice
On the American physics site Physics Stack Exchange experts and amateurs are also pondering this question. Most agree that although a battery theoretically becomes heavier when it is charged and lighter when it is empty, you cannot measure the effect in practice. ‘Just as you yourself become lighter when you blink your eye,’ one writes. ‘It costs energy and so you become lighter, but the effect is immeasurably small.’
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